Rapid Climate Change

Climate and Choices

Climate is the result of the exchange of heat and mass between the land, ocean, atmosphere, ice sheets and space. As long as changes to the land, ocean, atmosphere and ice sheets stay below the thresholds I have just described, climate changes will happen slowly. But the climate will change rapidly if those thresholds are crossed. So rapidly that it would be impossible to rearrange agricultural practices quickly enough to avoid stressing world food supplies. So rapidly that many species would not be able to adapt, because their habitat, already greatly reduced by human activities, would be eradicated.

Human ingenuity would most likely allow us to adapt to a rapid change in climate, but we would pay a larger price than our civilization has ever known. Imagine the economic and social cost of moving, in a 20-year period, most of our agricultural activities 500 miles south of their current locations. Imagine the social cost and famine if agriculture could not be relocated quickly enough. Even a short-duration event such as the Dust Bowl years in the 1930s had a large influence on American society. The Little Ice Age, which caused major resettlement in Europe in the 15th and 16th centuries, is a more likely analogue of where we might be headed.

Some have proposed that we could counterbalance the greenhouse effect by manipulating the global exchanges of heat and mass. Methods that have been discussed include blocking the Strait of Gibraltar to change the salinity of the North Atlantic, using airplane-distributed particles or large orbiting sunshades to shade the earth, and fertilizing the ocean with iron to promote production of carbon dioxide-consuming biomass. But we have a poor record of managing even small ecosystems and lack a complete understanding of the ocean-atmosphere interactions that govern our climate. Intentionally manipulating climate would not only be costly and imprecise; it would also be impossible to benefit some regions without adversely effecting others. It would be a risky experiment on the only planet we can call home.

Although we do not know the critical level of greenhouse-gas concentration, we do know that reducing the rate of greenhouse emissions would help in two ways. First, the atmospheric concentration of greenhouse gases would increase more slowly. Second, numerical models by Thomas Stocker and Andreas Schmittner of the University of Bern and others predict that the climate threshold will occur at a higher concentration of greenhouse gases if the concentration of greenhouse gases increases slowly. Slowing the rate of greenhouse-gas emissions would buy us more time to understand the consequences of our actions and might allow us to increase greenhouse-gas concentrations to a higher level before reaching the critical threshold.

It is true that computer models are not perfect; they indicate general patterns. And we need to improve our understanding in many areas before the models can pinpoint thresholds. For example, our understanding of the details of ocean circulation is poor, and the physics of cloud formation and their influence on heat exchange is elusive. When we model previous switches in climate, we can compare the model to the results of real-world experiments recorded in ocean sediments and ice cores. But when we model the future, we have no empirical basis to judge the model's accuracy. If we take no action until we are completely confident the models are correct, then the only use for the models will be to explain what happened. Our insistence on a tested model is part of the reason society is continuing to conduct the largest experiment ever done, the experiment of increasing the atmospheric concentration of greenhouse gases.

It will be another 20 years before the climate changes that are predicted to be associated with the greenhouse effect become large enough to be unambiguously differentiated from naturally occurring variations in climate. As a society we have the choice of ignoring the warning signs that our studies have uncovered or taking some action.

I think we should spend the next 20 years aggressively investigating our options. We should continue to focus on improving our ability to predict climate change. At the same time, we should test the technologies and polices we might need to reduce greenhouse-gas emissions, implementing them on a small scale where there would be minimal economic and social disruption. I am not alone among scientists in anticipating that 20 years from now our society may have to choose between disruptions associated with our current approach to energy use and disruptions associated with adopting an approach to energy use that produces fewer greenhouse gases. Procrastination will prevent making an informed decision and will increase the social and economic costs.